The present invention refers to a device for converting an electric current signal into a mechanical correcting variable, the device consisting essentially of a pneumatic correcting element with slide rod, a pneumatic amplifier, a compressed-air supply, and an electromagnetic signal converter which has a solenoid with coil, iron circuit and air gap, a nozzle/baffle-plate device and a lever which is mounted for swinging on a tensioning band. The lever bears the baffle plate on its one end and a permanent magnet arranged opposite the air gap of the solenoid on its other end. A distance between baffle plate and nozzle is variable in proportion to current flowing as input signal through the solenoid, and the correspondingly changed dynamic pressure in the nozzle are used, after pneumatic amplification, directly to control the pneumatic correcting element. A feedback of the mechanical correcting variable into the electropneumatic signal converter is provided.
For the same purpose of use, an electropneumatic signal converter having an electric measuring instrument and a pneumatic lagging device is known from Federal Republic of Germany AS 22 64 220, in which the coupling of the lagging device with the measuring instrument is effected via a nozzle/baffle-plate device. The correcting elements which bear the nozzle and the baffle plate are mounted in cross-spring joints, the axes or swing of which are parallel to each other and lie in the same plane. The known device operates in accordance with the path comparison principle. The stroke of the solenoid is converted into a movement of the baffle plate and the nozzle follows along in this movement by readjustment of the pressure.
One substantial disadvantage of this device is that the paths of a few tenths of a millimeter which can be obtained in the region of the nozzle must be compared with the path of the mechanical correcting variable, which lies within the range of 10 to 100 mm. Thus, a leverage of 1:100 to 1:1000 is necessary, which can be obtained only with very long levers or with an expensive mechanical transmission. In this way the apparatus becomes relatively large and expensive and its adjustment is very difficult. In addition, there is the fact that with such a leverage, considerable hysteresis errors are unavoidable, they being far above the value of 0.5% which is demanded today. Finally, in the known device relatively large masses must be moved so that the instrument is very sensitive to vibration and therefore scarcely enters into consideration for direct attachment to, for instance, process valves.